In the prior art, niobium cavities of the type well known and used in the operation of particle accelerators and the like apparatus have been fabricated by drawing and ironing of niobium sheet produced by cold rolling and annealing ingot produced material. While such material has proven satisfactory for use in niobium cavities, the material thus produced exhibits several shortcomings principally related to grain size and grain size distribution. Cold roll sheet material, for example, exhibits a relatively fine grain structure and thus a plurality of grain boundaries that affect its performance in cavity operation. Cold rolled sheet also exhibits significant variation in grain size through and along the length of the sheet material which also affects its performance in cavities. Cast niobium on the other hand exhibits large grain size and relatively uniform grain size distribution through the body of the material. The presence of large grains results in a reduction in the number of grain boundaries and hence enhanced performance in the final cavity structure. Thus, if the relatively large and uniform grain size characteristics of the “as cast” or ingot niobium could be preserved in the formed cavity, performance would undoubtedly be improved.
In addition to the above described grain size related shortcomings of the prior art sheet material based cavities, there are other significant shortcomings associated with the use of cold rolled sheet material in the fabrication of niobium cavities. Among these are: 1) the costs associated with cold rolling and annealing of niobium to produce sheet are relatively high; 2) because of the relatively small grain sizes exhibited by cold rolled sheet materials, their strength when heated can be unacceptably reduced; and 3) cold rolled sheet demonstrates “memory” or “springback” characteristics that may require extensive and expensive finishing of the formed cavity after drawing and ironing to assure accurate dimensional characteristics. Such springback is due to the presence of banding or a lack of homogeniety of grain size in the cold rolled sheet. All of these shortcomings can be positively affected through the use of “as cast” ingot based niobium starting materials that possess large and relatively uniform grain size distributions.